This relates to data communication equipment and in particular to a data receiver particularly useful in a data communication system in which more than one data communication message may be provided simultaneously to the same data transmission line.
U.S. Pat. No. 4,063,220 describes a multipoint data communication system, commonly referred to as an "Ethernet" system, in which two or more data processing stations are interconnected by a common communication cable. At each station a tap in the communication cable is connected by a transceiver to an appropriate interface to station equipment. The communication system permits each station to access the data communication cable so as to transmit messages to other stations connected to the same cable. Because the stations are independenty operated, more than one station may attempt to use the cable substantially simultaneously. Equipment is provided, however, in each transceiver to detect such a data collision between a signal being transmitted from that transceiver and a signal already on the communication cable and to abort the data transmission.
Numerous techniques are known in the art for detecting a data collision. In the '220 patent, an exclusive OR gate compares the transmitted signal with the signal received from the communication cable and aborts transmission when they are not the same. Another technique is to monitor the DC level of the received signal and, if this level is higher than a specified threshold, assume there is a data collision and shut down the transmitter. Alternatively, as disclosed, for example, in U.S. Pat. No. 4,282,512, the receiver can look for data transitions that occur at timings different from those expected for the received data.
All of these techniques have their limitations. They miss collisions that start simultaneously or within the timing tolerances allowed for by the receiver. They also cannot detect very low signal levels that may well occur if the colliding data stream has been transmitted a relatively long distance and/or through several repeaters. Other difficulties with presently available receivers include unduly complicated collision detection circuitry, ambiguities in the turn-on state of this circuitry which may be randomly high or low and the inability to comply with standards such as the single power supply requirement of the Ethernet standard.